This invention relates to the generation of electrical power using piezoelectric materials, and particularly to such power generation using field-induced piezoelectricity within electrostrictive materials such as polyurethane.
It is now well known to use piezoelectric materials in various apparatus for the conversion of natural sources of mechanical energy, e.g., surface waves on oceans, directly into electrical power. A characteristic of certain piezoelectric materials is that they are internally permanently poled, i.e., they are xe2x80x9cferroelectricxe2x80x9d materials, possessing an intrinsic electric field which can be altered by strainings of the materials and attendant electric charge movement and electrical energy generation.
Presently, a preferred piezoelectric material for many power generating applications is the manufactured polymer, PVDF. As normally made, PVDF is not a piezoelectric material. However, ferroelectric characteristics can be effectively permanently induced therein by heating the material while in a d.c. electric field for causing the electric dipoles of the material to align with the field, and then slowly cooling the material, while still in the field, for xe2x80x9cfreezingxe2x80x9d the dipoles in place.
Aside from being ferroelectric, PVDF has several other characteristics making it particularly suitable for use in electrical power generators. For example, being a plastic-like material, it is relatively inert (thus, safely usable in corrosive, e.g., ocean environments); it is relatively easily strained (for utilizing the available mechanical energy most efficiently for altering the material internal electric field); and it can be repeatedly mechanically strained without loss of desired characteristics. It has other desirable characteristics as well.
Depending upon how the PVDF material is used, calculations and experimental data show that mechanical to electrical energy conversions can have efficiencies as high as 30%. While impressive, room for improvement exists.
The inventors herein are aware of another polymer transducer material; namely, polyurethane. By xe2x80x9ctransducerxe2x80x9d is meant that polyurethane is xe2x80x9celectrostrictivexe2x80x9d and can be elastically deformed by an externally applied electric field. However, polyurethane and other electrostrictive materials are not ferroelectrics in that they contain no intrinsic internal field and, unlike PVDF, they can not be treated to obtain a permanent poled condition. To the inventors"" knowledge, electrostrictive materials have never been considered for use for power generation purposes. One reason, perhaps, relates to earlier uses of ferroelectric materials. Materials such as PVDF have long been used in mechanical energy detectors, e.g., for underwater detection of sound energy. A sound detector made of an electroded sheet of poled PVDF is, without more, a sound energy detector. Varying intensity sound pressures, even of extremely small amplitude, generate corresponding a.c. voltages across the PVDF sheet which are collected by the electrodes for electronic amplification. Familiarity with such signal detecting devices is one reason why PVDF was considered for use in power generating applications.
Conversely, the strain versus electrical charge characteristics of electrostrictive materials are such that they are quite inefficient for use in direct substitution for PVDF type materials in the aforedescribed signal detection applications. Thus, they were never so used, and not later considered for use in power generation applications. Conversely, electrostrictive materials, and polyurethane in particular, have been extensively used in xe2x80x9cactuatorxe2x80x9d applications, i.e., electrically operated transducers for providing precise, small mechanical movements in response to applied electrical control signals.
The herein inventors have experience both with PVDF power generators and with polyurethane actuators. It became evident to them, based upon their evaluations of those parameters of PVDF which contribute to, or detract from, the utility of PVDF as a power generator, that polyurethane has many physical characteristics which at least suggest that polyurethane would be more efficient than PVDF for power generation applications. Both the recognition of the suitability of polyurethane (as well as other electrostrictive materials) for use as a power generator, and the means required to make such use both possible and practical, constitute the present invention.
For providing a body of an electrostrictive material, e.g., polyurethane, with piezoelectric characteristics, electrodes are provided on spaced apart surfaces of the body, and a d.c. voltage is applied between the electrodes for establishing an electric field through the body. The mechanical energy to electrical energy conversion of such materials (i.e., a parameter known as d31) is directly proportional to the bias field. For maximum power conversion efficiency, the d.c. voltage is as high as possible consistent with reliable use of the power generator. Other parameters of electrostrictive materials affecting their suitability for power generation are discussed hereinafter.